This invention relates generally to irrigation equipment and more particularly to a center pivot irrigation machine which is provided with a swing arm attachment for irrigating corner areas of fields.
Although center pivot type irrigation machines have achieved considerable popularity and function satisfactorily for the most part, they are capable of watering only circular areas. The basic center pivot machine is not able to irrigate the corner areas of square fields or other areas outside of the circle covered by the machine. Various types of attachments have been proposed to more adequately cover the sizeable land area represented by the field corners. To date, the most successful type of "corner system" is known as a swing arm or swing span.
The swing arm is essentially an end span similar to the remainder of the irrigation machine except that it has a hinge connection with one end of the main boom and is supported by a tower having steerable wheels. As the main boom turns about the central pivot stand, the steerable tower follows a buried cable or other guide device which causes the swing arm to pivot out into the corners of the field and to retract back behind the main boom as the system departs from a corner. In this manner, the swing arm system provides an extension boom which is able to cover a good part of each corner of the field. The known prior art relating to swing arm corner systems includes U.S. Pat. Nos. 3,797,517 to Kircher et al, 3,802,627 to Seckler et al and 3,902,517 to Daugherty et al.
All of the swing arm corner systems that have been proposed in the past fail to apply water in a uniform manner to the corners of the field. As the swing arm folds out into the corner, it travels faster than the system as a whole, and the area covered by the swing arm during its extension is underwatered. Conversely, the swing arm moves relatively slowly as it retracts out of the corner, and over watering occurs in the areas covered by the swing arm as it folds back behind the main boom. The overall result is that some areas near the field corners are underwatered and other areas are overwatered, and the effectiveness of the irrigation suffers accordingly.
The uniformity of the water pattern also suffers due to the changes that occur in the effective overall length of the irrigation system as the extension boom folds out and then back in. In order to apply water evenly, it is necessary to supply water to the machine at varying rates because much more water is required when the machine is operating at its maximum length than when it is at minimum length. For example, if the effective length of the system is 1280 feet with the swing arm fully retracted and 1545 feet with the swing arm fully extended, the water requirements are over 30% less in the retracted mode of operation than in the fully extended mode (when an end gun is included).
The need to provide varying amounts of water to the system detracts from the efficiency of the pumping equipment and the supply network. Also, the water supply must operate at much less than full capacity the majority of the time since full capacity is required only when the swing arm is fully extended. Even if the water supply has sufficient capacity to irrigate the total coverage area of the irrigation system, it may nevertheless be inadequate in actual practive because it must operate most of the time at 70% or less capacity. This makes many marginal wells impractical for use with the irrigation machine.
Efficiency of swing arm machines is dependent largely upon accurate guidance of the extension boom and effective control of the steering action and the speed of both the main and the extension booms. In the past, the extension boom has been attached to the end of the main boom by means of a hinge that permits the extension boom to swing in and out and a separate sliding mechanism that allows relative linear motion parallel to the extension boom. The linear motion is needed to operate switches that stop the tower of the swing arm when it moves too far ahead and to stop the last tower of the main boom if necessary to permit the swing arm tower to catch up. In addition to the cost and difficulty involved in constructing a suitable sliding mechanism, it is not uncommon for devices of this type to either slide erratically or completely bind. Consequently, the control arrangement operated by the sliding mechanism is subject to malfunction.
The end boom section which extends beyond the tower of the swing arm is typically supported by rods or cables extending to its outer end from a mast forming a vertical extension above the steerable tower. It is necessary to anchor the mast in a vertical position by supports such as truss rods or cables. If the boom section is relatively long and heavy, the truss rods or cables distort the shape of the swing span. In order to avoid this distortion, the rods or cables are sometimes extended back the full length of the swing span, thus increasing the material costs and detracting from the appearance of the structure.